Need help with Categories and Posts not being created

Please note; I have WP 3.03, but it wasn’t available as a choice.

My problem is that I’ve been trying to create new categories, both by writing new posts and trying to create a new category to put the post in, and also by just trying to create a category using Posts/Categories but neither is working.

This is the error message I get: “Fatal error: Cannot use object of type WP_Error as array in /home/johnmich/public_html/guitar-traders.com/wp-admin/admin-ajax.php on line 242”

I tried doing a new install on WP thinking that might be the problem since it is showing an error in an admin file, but to no avail. I’m using Firefox with the latest version of WP, and I’ve tried two different themes and run a check on the database, but nothing has worked, so I could use some help.
Cheers
John